The invention relates to a heat shield device suitable for use with a soldering device.
A soldering device is known from EP 0546781, which uses cylinders to transport gas through a hollow stem to produce a flame at an orifice (flame delivery cylinder) to be used as the heat source, or to feed the solder wire through the hollow stem of a second cylinder (wire delivery cylinder) to be melted by the heat energy supplied by the flame to the spot where the soldering is to take place.
The hollow stem cylinder based flame delivery system offers great advantages in delivering the flame to spots with restricted access and in the simplicity of the flame transport from a xe2x80x9cHomexe2x80x9d (or standby) position to the xe2x80x9cWorkxe2x80x9d position.
The flame has by its nature an elongated shape in the direction of the flame nozzle and accordingly its maximum heat is in the direction of the cylinder stem movement. That means that the travel distance between the cylinder Home position and Work position must be physically long enough to not have in the standby position a constant heat influx (obviously reduced as distance increases) on the job area.
The flame is preferably generated using a mixture of hydrogen and oxygen. This gas mixture may be made by electrolysis from water in which case the mixture is 2 parts hydrogen to 1 part oxygen. Flames of this kind have almost no side heat radiation but an almost needle sized elongated heat shape.
The distance between Home and Work positions must be, as an example for a nozzle with inner diameter 0.8 mm and a gas pressure of 100 mBar, around 300 mm in order to provide adequate heat isolation of the job area in the Home position. At that distance the heat is still so intensive that a hand palm must be quickly withdrawn to avoid burns. On the other side it is possible to approach the flame shape near the nozzle to almost a few millimetres before some heat is sensed.
This flame behaviour obviously provides advantages for working in tight spaces as long as the flame can be retracted sufficiently after the job is done.
The long travel distance required for heat isolation has several negative technical points of which the major ones are listed here:
It will take longer for the flame to travel from Home to Work, therefore adding additional time to the cycle time needed to perform a soldering task,
Cylinders with longer stroke are more costly,
Design space in an assembly system needed in the axial direction of the cylinder is long,
Cylinders with longer stroke tend to have the following problems:
a less stiff hollow stem,
pointing less precise to target,
end of stem tends to vibrate relative to the target on stopping in the Work position.
The stem thickness could be increased to give a higher stability. This again makes the cylinder heavier, and requires a wider design diameter, therefore reducing the access to tight spots.
Solutions for the heat isolation problem have been found and are applied in practice, but with a relatively substantial additional use of mechanical parts and movement controls as described below.
There exist four basic solutions:
1. Switch off the flame when the flame cylinder arrives back in Home position. Cutting the gas flow by closing a gas valve in the gas flow path can be done, thereby extinguishing the flame. For the next operation the flame will have to be reignited. This means an automated (electric/electronic) ignition device properly sequenced with the valve ON (gas flow) function and an eventual flame check before operating the task (since reignition could be unsuccessful at the first attempt). There is a control oscillation for the generator gas flow controller between OFF (no gas production) and ON (gas production adjusted automatically to fit the gas rate needed to establish the gas pressure). The constant on/off of the flame will place additional thermal wear on the nozzle outlet orifice. Each time the flame pops out the high temperature flame will fall on the nozzle outlet orifice and heat it there. A hydrogen/oxygen flame with properly adjusted gas pressure will not heat the nozzle end as long as the flame is on.
2. Slide a heat shielding mask from the side into the flame heat direction path, thereby shielding the target area from the heat source. There could be provided an additional properly timed cylinder with a mounted heat shield. However, the shield cylinder would have to be synchronized with the movement of the flame cylinder, and collisions could occur.
3. Move the flame cylinder axis, so the flame direction points to another heat absorbing area. eg a sort of chimney to diffuse the heat. The flame cylinder would have to be mounted with a bearing at one end so it can swivel around that point, and could be moved using a pivot cylinder.
4. Move the job target out of the flame heat direction.
The preferred embodiment of the present invention provides a heat shield device comprising a heat shield and means for moving the heat shield to and from a shielding position in which it shields, in use, a target area from a heat source, wherein the movement of the heat shield is mechanically linked to a movement of the heat source away from and towards the target area.
The means for moving the heat shield preferably include means for providing a biassing force.
The means for providing the biassing force preferably bias the heat shield away from the shielding position. The biassing means may comprise a resilient member by which the heat shield may be secured with respect to the heat source. Alternatively the heat shield may be pivotably mounted with respect to the heat source, the pivoting action being influenced by spring means, for example.
Additionally or alternatively, the means for moving the heat shield may comprise a cam surface provided on the heat shield device which cooperates with a surface which is fixed with respect to the heat source. Preferably said cooperating surfaces provide a camming action to move the heat shield into the shielding position.
The invention also provides a heat delivery device provided with a heat source and a heat shield device as described above. In the case of a heat shield device provided with a cam surface, the heat delivery device preferably has a cooperating surface in the form of a protruding shoulder. Preferably the cam surface on the heat shield device faces the workpiece and the other cam surface faces away from the workpiece.
The device according to a preferred embodiment of the invention is a very simple low cost solution to the problem of isolating the target area from the heat source. It is particularly advantageous that the shielding of the heat source is automatically synchronised with the movement of the heat source away from the target, without the need for external control systems.
An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which like numerals refer to like parts throughout and in which: